The goal of this research is an understanding of the action of near- ultraviolet radiation (near UV; 300-380 nm) on living cells. This radiation is present in sunlight. In general, it is not absorbed by, and does not directly damage, nucleic acids, as does far-ultraviolet radiation (far UV; below 300 nm). The most sensitive action of near UV on living cells is inhibition of growth. This effect has been observed in a wide variety of living systems, and it is generally temporary. The central approach of our research has been to investigate a variety of effects of near UV on bacteria, and to attempt to relate these effects to the mechanism of growth inhibition. This, of course, does not imply that, if we find an effect not related to growth inhibition, further research is not warranted; we simply use growth inhibition (or growth delay) as a point of departure. Specific aspects of our research can generally be classified into one of five categories: (a) near UV as an ecological factor (b) actions of near UV on oxidative phosphorylation (c) actions of near UV on membranes (d) actions of near UV on DNA (e) repair of DNA by near-UV treatment. The last category includes the phenomena of photoreactivation and photoprotection, study of which was a major part of our early effort, but which is only a minor fraction of the present and proposed future research.